Along with the development of digital technology, recent years have seen a further increase in functionality of electronic devices such as mobile information devices and information home appliances. The increase in functionality of these electronic devices is accompanied by rapid progress in the miniaturization and speeding-up of semiconductor elements used therein. Among these, a large-capacity nonvolatile memory represented by a flash memory is rapidly being given wider application. Furthermore, as a next-generation nonvolatile memory of a new type which will replace this flash memory, a nonvolatile memory device which includes, what is called, a variable resistance nonvolatile memory element has been progressively studied and developed. Here, a variable resistance nonvolatile memory element is an element whose resistance value reversibly changes based on electrical signals, and is capable of storing data corresponding to this resistance value in a nonvolatile manner.
As disclosed by Patent Literature (PTL) 1, a variable resistance nonvolatile memory element has a structure in which a variable resistance layer made of a variable resistance material is located between paired electrodes, and is roughly classified into two types, a bipolar type and a unipolar type, based on a difference in electrical properties.
The bipolar nonvolatile memory element (hereinafter referred to as a bipolar element) is an element of a type that requires voltages different in polarity for changing its resistance state from a high resistance state to a low resistance state (low resistance writing) and for changing its resistance state from a low resistance state to a high resistance state (high resistance writing). In contrast, the unipolar nonvolatile memory element (hereinafter referred to as a unipolar element) is an element of a type that requires voltages the same in polarity for low resistance writing and for high resistance writing.
As disclosed by PTL 2, stable high-speed drive at low voltage can be realized using a bipolar element which includes a variable resistance layer having a structure in which a high resistance layer (a high-oxygen-concentration layer) and a low resistance layer (a low-oxygen-concentration layer) are stacked. PTL 2 discloses a variable resistance element in which tantalum oxide layers having different oxygen content atomic percentages are stacked as a variable resistance layer.